1. Field of the Invention
The present invention relates generally to the field of imaging a two-dimensional field of regard. More particularly, the present invention relates to imaging of the full-Earth disk by a spacecraft that scans the field of view of an imager across the full Earth disk.
2. Background Information
One of the most common uses for artificial planetary satellites is to produce images of the planet""s surface. Many Earth-orbiting satellites capture images of Earth for purposes ranging from military intelligence to weather forecasting. Orbital imaging for weather forecasting and for scientific purposes demands images of vast areas of the Earth at once.
It is common to image a planet (e.g., Earth) from space using an imaging electro-optical sensor constructed from a telescope that collects radiation from a remote source and brings it to focus on one or more focal plane arrays (FPA""s) with each FPA containing many detector elements. A scanning sensor moves the image of the scene over one or more FPA""S, each FPA usually having many detector elements perpendicular to the direction of the scan motion. Each element converts the radiation from an instantaneous field of view (IFOV) in the scene into an electronic signal. The image of the scene in the spectral band of the sensor is reconstructed from these electronic signals.
The angular field of view (FOV) of the telescope multiplied by its effective focal length (EFL) equals the dimensions of the telescope""s focal plane, which contains the FPA. For application in which an imaging sensor must cover a two-dimensional field of regard (FOR) that exceeds the telescope""s FOV, a plane scan mirror may be located in front of the imaging sensor""s telescope to scan the FOV across the FOR. For example, the Earth subtends a circle approximately 17.4xc2x0 in diameter from geosynchronous altitude. An instrument that is capable of imaging the full-Earth disk must have a field of regard (FOR) that not only includes this full-Earth disk, but also allows it to view deep space to measure the background signal in each channel. Most multispectral instruments that image the Earth from this altitude use a large, reflective telescope with a field-of-view (FOV) that is much smaller than the required FOR. A two-dimensional raster scanning procedure is required to cover the FOR, and is usually implemented with a plane mirror in front of the telescope""s aperture.
A number of geosynchronous weather satellites, including the EUMETSAT and GOES-1 through GOES-7 satellites, are xe2x80x9cspinnersxe2x80x9d that rotate about the north-south axis. The imager on each of these spinning satellites has a telescope that is aligned with the north/south axis of the spacecraft. A plane mirror with a single rotational axis, perpendicular to the spin axis, reflects the optical axis of the telescope towards the Earth. The spacecraft""s rotation scans the line of sight (LOS) in the east/west direction. To form a two-dimensional map of the Earth, the plane mirror is only required to step in the north/south direction. The main disadvantage of a spinning satellite is that it only allows the imager to view the Earth""s surface for less than 5% of its total duty cycle.
Beginning with GOES 8, the geostationary weather satellites operated by the United States (developed for NOAA by LORAL with instruments from ITT) have been three-axis stabilized. In this configuration, the imager continuously points toward the Earth, permitting it to operate at a high duty cycle and to be far more flexible than a spinning satellite in imaging arbitrary areas of the Earth""s surface. These prior art GOES imagers routinely produce 3000 km by 5000 km images of the contiguous United States (CONUS) and 1000 km by 1000 km images of severe storms. Scanning is performed by a plane scan mirror mounted on a two-axis gimbal. Rotation of this mirror about the inner gimbal axis scans the LOS in the east/west direction. Between scan lines, incremental rotations about the outer gimbal axis move the LOS from north to south. When scanning the Equator, the GOES imager projects its detector arrays onto the Earth""s surface in the optimal manner, with the cross-scan axis of each detector array (its long axis) projected in the north-to-south direction. When scanning north of the Equator, the projection of this axis is tilted in the northeast-to-southwest direction; when scanning south of the Equator, the projection is tilted northwest-to-southeast. The tilt angle varies from zero at the Equator to 8.7xc2x0 at the North and South Poles. This phenomenon, known as image rotation, is an intrinsic problem in a two-axis scanning system that uses a single scan mirror in object space. See J. J. Shea, xe2x80x9cImage correction via lunar limb knife-edge OTF""sxe2x80x9d, Proc. SPIE, Earth Observing Systems III, vol. 3439, Jul. 19-21, 1998, pp 165-186.
Referring to FIG. 1, the geometric configuration of the GOES 8 and 9 imager is illustrated. The GOES convention for spacecraft coordinates is portrayed by a set of orthogonal coordinates 10 wherein +x=east, +y=south, and +z=nadir. For simplicity of illustration, the GOES telescope is represented as a single lens 12 and crossed axes 14 represent the image as presented at the focal plane array. Note that the lens 12 inverts the image of the axes 14. The telescope""s optical axis 16 points due east along the x-axis. The scan mirror 18 is an optical flat with an elliptical cross section and has a reflective surface (not visible from the viewpoint of FIG. 1) that directs light from the Earth (shown in phantom) into the telescope 12. The scan mirror 18 is mounted on a first axle 20 that provides an inner axis of rotation with respect to which the inner gimbal angle (iga) is measured. The inner axis of rotation is coincident with the short dimension of the ellipse and perpendicular to the normal vector of the mirrored surface. The first axle 20 permits the scan mirror 18 to rotate about the inner axis of rotation with respect to a yoke 22. The yoke 22 has a second axle 24 that is perpendicular to the first axle 20. The second axle 24 lies along the extension of the optical axis 16 of the telescope 12, along the x-axis, and allows the yoke 22 to rotate about this outer axis of rotation that is fixed with respect to the telescope 12, and with respect to which the outer gimbal angle (oga) is measured. The orientation of the first axle 20 always remains perpendicular to the x-axis, but rotates in the y-z plane when the yoke 22 is pivoted about the second axle 24.
Referring to FIG. 2, projections 42, 44, 46, 52, 54, 56 of the crossed axes in the focal plane 14 onto the Earth""s surface 30 are illustrated. The line with the arrowhead 14xe2x80x2 is parallel to the z-axis and corresponds to the along-scan direction of the array. The line with the circle 14xe2x80x3 is parallel to the y-axis and corresponds to the cross-scan direction of the array.
The intersection of the crossed axes is projected onto the equator 32 when the position of the yoke 22 on the outer axle 24 aligns the inner axle parallel to the y-axis. This angle can be defined as the home position of the outer axle, at oga=0. When the outer axle 22 is fixed at this position and the scan mirror 18 is rotated about its inner axle 20, the projection 42, 44, 46 of the focal plane 14 is scanned along the equator 32. The y-axis of the focal plane remains perpendicular to the direction of the scan and the z-axis of the focal plane is projected along the direction of scan.
When the yoke 22 is rotated about the outer axle 24 in the +oga direction, the crossed axes in the focal plane 14 are projected 52, 54, 56 into the Northern Hemisphere. The array""s projection 52, 54, 56 rotates clockwise, as viewed from space. When the oga remains fixed and the scan mirror 18 is rotated about the inner axle 20, the z-axis of the focal plane is no longer projected along the direction of scan but it is tilted away from this position by an angle equal to oga, reaching an angle of 8.7 at the North and South poles. When the oga less than 0 (not shown), the focal plane 14 is projected into the Southern Hemisphere with a counterclockwise rotation equal to the magnitude of the oga.
The GOES 8 and GOES 9 imagers scan the Earth in a series of alternating east-to-west and west-to-east lines proceeding from North to South. The GOES 8 and 9 imagers only maintain the optimal relationship between the direction of the opto-mechanical scan and the orientation of the array when they scan the equator. However, rotation of the mirror about a second axis rotates the projection of the focal plane onto the Earth""s surface. This is a disadvantage of the prior art because it precludes use of a resolution enhancing technique known as Time Delay and Integration (TDI). The fundamental aspects of TDI technique are explained as follows, along with an explanation of why TDI is incompatible with the prior art systems that suffer from image rotation.
TDI is a prior art technique in which a two-dimensional detector array is scanned like a linear array. TDI achieves the angular resolution of a single detector element in the array combined with a dwell time that is N times longer, where N is the number of columns in the along-scan direction. Referring to FIG. 3, TDI is illustrated, showing the projection of a 4xc3x974 array onto object space, with the solid square, ▪, representing a single charge packet. One each cycle, the charge packet in each detector element of the array accumulates photo-electric charges generated by absorption of radiation from the scene and is then transferred to the detector element in the same row that lies immediately to its right. The column on the far right of the array is also read out at the end of each cycle. At the end of cycle 4, the charge packet represented by the solid square contains the sum of the charges accumulated by the four detector elements in the second row: column: #1 (the far left column) in cycle 1, column #2 in cycle 2, column #3 in cycle 3, and column #4 in cycle 4.
The opto-mechanical scan motion of the LOS must be coordinated with the electronic scan of the charge packets so that the LOS from each charge packet corresponds to a single pixel in object space. The linear velocity of the electronic scan, from the leading edge to the trailing edge of the array, must match the linear velocity of the opto-mechanical scan. Referring to FIG. 4,the projection of the row or detector elements in object space is illustrated, where the right-to-left opto-mechanical scan cancels the left-to-right electronic scan, so that the image of a charge packet remains at a fixed angle in object space, corresponding to a fixed IFOV on the Earth""s surface. The effective dwell time is equivalent to the dwell time of a single IFOV multiplied by N (N=4 in this example), so that radiometric signal-to-noise ratio (SNR) is enhanced by a factor of N1/2 (2 in this example) in comparison to a single column FPA scanned at the same opto-mechanical rate.
The goal of TDI is to accumulate all of the photo-electric charges in each charge packet from the same instantaneous field of view (IFOV) on the Earth""s surface. To achieve this condition, the LOS of the present invention is scanned opto-mechanically so that the projection of the array moves horizontally, from left-to-right, with its velocity parallel to the rows of the array. At the same time, the array is electronically scanned so that a charge packet moves along each row from right-to-left. The electronic and opto-mechanical scan vectors are substantially equal in magnitude and opposite in direction to maintain image quality.
To achieve the desired match between opto-mechanical and electronic scans, two conditions need to be satisfied for all scan angles within the FOR. Ideally, the projection of the TDI axis of each FPA onto the Earth""s surface must always lie along the direction of the opto-mechanical scan. Without this provision, the pixels in the image will be blurred in the cross-scan direction. Also according to the ideal case, the angular scan speeds of the LOS due to the electronic scan and the optical rate and the scan rates must be also equal. Without this provision, the pixels will be blurred in the along-scan direction. If both of these conditions are satisfied, then the outputs from the array will have the angular resolution as a single detector element but an effective dwell time per pixel equal to N times the integration period of a single cycle, where N is the number of columns summed in the TDI (N=4 in the illustrated example).
Thus, in order to obtain the potential advantages of the TDI technique, the image rotation problem of the above-described prior art imaging system needs to be solved.
Another problem with the prior art has to do with how errors are inherently introduced across each scan line by variations in emissivity of the scan mirror. The GOES imager measures its IR background by viewing deep space on one side of the Earth""s surface at the end of each scan line. This background is subtracted from the raw scene measurements to determine the net radiance from the scene. Unfortunately, the emissivity of the GOES imager""s scan mirror varies as a function of the angle of incidence and this angle of incidence varies by about 9.5xc2x0 on each scan line. Because the background from the scan mirror is not constant during a scan line, the raw data exhibits east/west shading with errors of several degrees Kelvin in data from the Earth""s surface. The error caused by this variation in emissivity over a single scan line is reduced, but not eliminated, by calibration. See M. Weinreb, et. al, xe2x80x9cOperational calibration of Geostationary Operational Environmental Satellite-8 and -9 imagers and soundersxe2x80x9d, Applied Optics, vol. 36, no. 27, Sep. 20, 1997, pp 6895-6904.
Referring to FIG. 5, the projection of a 2-D FPA into object space on alternate scan lines in a bi-directional scan is illustrated. The FPA is fabricated with a read-out column on each side of the active photo-detector array. Reversing the phasing of their charge-coupled transfer reverses the direction of motion of the charge packets in a row. These bi-directional TDI arrays permit bi-directional scanning, so the end of one scan line in an image can be in close angular proximity to the beginning of the next scan line. Between these two scan lines, both the opto-mechanical and electronic scan directions along the TDI axis must be reversed and the optical LOS must be offset in the cross-scan direction by an angle equal to (or slightly less than) the cross-scan angular subtense of the array.
There are other applications beside TDI that require that a sensor""s opto-mechanical scan motion to lie along a fixed axis of a focal plane. For example, a multispectral scanner may have a focal plane that contains a series of linear FPA""s, arranged as columns in the focal plane, each with a unique spectral filter. To achieve co-registration among corresponding detector elements in several spectral channels, the same IFOV in the image must be scanned over the corresponding detector element in each FPA.
In prior scan mirrors on gimbal systems, including the GOES 8/9 imager, there is a variable relationship between the direction of scan produced by rotation about a single axis and the projection of the axes of the focal plane. In order to scan the image at a constant velocity (speed and direction) in the focal plane, it would be necessary to perform simultaneous, coordinated scanning at variable rates about the two gimbal axes. The outer gimbal must be capable of scanning over the full range of angles required to cover the FOR, continuously or in increments that are small in comparison to the IFOV of the sensor.
When the mirror is rotated about its inner axis, it changes the moment of inertia of the mass that is being rotated about the outer gimbal axis. Also, the outer gimbal axis cannot be a principal axis of the system for all mirror angles unless the mirror motion is compensated by a counter-rotating mass.
In most gimbal implementations, the outer gimbal must support the motor that drives the inner gimbal""s rotation as well as an encoder or a resolver to make precision measurements of the inner gimbal""s angle. Electric power and signals must be transferred cross the outer gimbal axis (e.g., by flexible cable or slip ring), increasing the rotational friction about the outer gimbal axis. Since highly accurate pointing is required, it is difficult to implement the necessary scan pattern by simultaneous rotations about both axes during the active portion of the scan.
Thus, what is needed is a scanning imager that avoids the problem of image rotation so that TDI may be effectively used. What is also needed is a scanning imager that has a reduced variation in emissivity of the scanning optics across each scan line. What is also needed is a scanning mechanism that requires rotation about only one axis, preferably the inner axis, during the data-taking portion of the scan pattern.
The subject invention is a method and apparatus for scanning a two dimensional field of regard with a single plane mirror in the object space of a telescope, maintaining a fixed relationship between the rotational direction of scan and the projection of the telescope""s focal plane. The two dimensional FOR is covered by a series of conical arcs, each arc being scanned by rotation at constant angular velocity about the inner axis of the two-axis system. This scanning system can accommodate applications such as TDI that require an opto-mechanical scan with a constant linear velocity (magnitude and direction) in the focal plane.
Shading of IR images is particularly hard to correct because the observed radiance has a component that varies with both the temperature of the scan mirror and its reflection angle. Calibration measurements, determined by viewing a source of known radiance, can mitigate this problem, but the time between calibration and scene measurements must be as short as possible or the calibration will be degraded by thermal drifts and 1/f noise in the detector and electronics.
In the subject invention, the reflection angle""s dependence of the rotational angle reaches a minimum at the center of each arc in the FOR, and exhibits only a slow, quadratic increase towards each end of the arc as a the scan mirror is rotated about the inner gimbal""s axis. The large variations in scan angle occur between arcs rather than within an arc. In the geosynchronous imager application, measurements of dark space, taken at the ends of the scan line, are used to subtract the instrument""s background from the raw signals. The space measurements taken on each side of each arc may be used for calibration of that arc, mitigating the shading problem. It is also an option to place one or more calibration sources, such as blackbodies with precision temperature-monitoring apparatus, in positions where they can be viewed between scan lines.
The only rotational motion during the active arc scanning is the rotation of the mirror at a constant angular velocity about its principal axis of minimum angular momentum. Because the outer axis of rotation remains fixed during the active portion of the scan, dynamic errors (jitter) in the outer gimbal""s rotation and cross-coupling between the rotational motions about two axes are eliminated. Because the angular velocity is constant, torque disturbances due to angular acceleration about the inner axis are also eliminated during the active portion of the scan. All of these factors tend to reduce or eliminate errors in the pointing accuracy of the system.
Between these arcs, the orientation of this inner axis is offset by rotation about the outer gimbal axis. The outer gimbal needs only to be capable of holding a small number of fixed positions, with the mechanical angle between positions no greater than one-half of the optical width (cross-scan) of the scanned arcs. These factors tend to simplify the apparatus required to measure and control the rotational angle of the outer axis. The position of the outer gimbal must be-known to the same level of accuracy as that of the inner gimbal, however. The angular velocity of the mechanical scan is held constant during any given arc. However it is preferable to change the angular velocity from arc-to-arc, in order to maintain the same optical scan rate for each arc.
It is an object of the present invention to provide a scanning imager that avoids the problem of image rotation so that the along-scan and cross-scan axes of the scan pattern correspond to fixed axes in the focal plane throughout the field of regard.
It is also an object of the present invention to provide a scanning imager that substantially eliminates image rotation so that TDI may be effectively used.
It is another object of the present invention to provide a scanning imager that has a reduced variation in emissivity of the scanning optics across each scan line.
It is yet another object of the present invention to provide a method for scanning an imager that compensates for the problem of image rotation.
It is still another object of the present invention to provide an imaging satellite that images a planet""s surface while compensating for the problem of image rotation.
It is a further object of the present invention to provide a scanning imager that allows for effective co-registration among multiple detector arrays.
It is an additional object of the present invention to provide a scanning imager that permits multi-spectral imaging using multiple detector arrays.
It is another object of the present invention to provide a scanning imager that permits hyperspectral imaging.
It is also an object of the present invention to provide a scanning imager that allows the outer axis of a two-axis scanning gimbal to remain stationary during the data-taking portion of the scan pattern.
Some of the above objects are achieved by a method of scanning a field of view of an imager across a field of regard using a scan mirror mounted on a gimbal having an inner axis and an outer axis. The method includes sweeping the field of view across the field of regard in a selected direction by rotating the gimbal about the inner axis while maintaining the gimbal at a fixed angle with respect to the outer axis. The method further includes progressing to a subsequent scan position by rotating the gimbal about the outer axis by a predetermined increment angle while maintaining the gimbal at a fixed angle with respect the inner axis, Additionally, the method includes repeating the act of sweeping such that the selected direction is chosen alternately from a first direction and a second direction that is opposed to the first direction. The method further includes repeating the act of progressing prior to each repeated act of sweeping, wherein there is substantially no rotation, with respect to the instantaneous direction of scan, of an image formed on the imager.
Others of the above objects are achieved by an apparatus for scanning a two dimensional field of regard. The apparatus includes a telescope having a focal plane and a field of view, and one or more image sensors disposed at the focal plane. It also includes a single optically flat mirror disposed in the object space of the telescope, wherein the flat mirror scans the field of view across the field of regard while maintaining a fixed relationship between the rotational direction of scan and the projection of the telescope""s focal plane.
Some of the above objects are obtained by an apparatus for imaging a two dimensional field of regard. The apparatus includes an imager having a field of view along a line of sight, the field of view being substantially smaller that the field of regard, as well as a scan mirror disposed so as to cast the line of sight onto the field of regard. The scan mirror causes the line of sight to be scanned across the field of regard in a conical arc.